Robotic manipulators can be found performing numerous industrial applications. Notably, such manipulators have also been used in orbit during space shuttle missions for tasks such as satellite repair, and will also be used throughout the construction of the planned International Space Station. Lightweight robotic manipulators, as shown in, for example, U.S. Pat. No. 6,002,232 to McConnell et al., can offer power savings for industry and space applications since less manipulator mass is accelerated during motion. These manipulators, however, often have flexible members (e.g., arms) that are susceptible to unwanted vibration or oscillation during motion. Any unwanted oscillations that increase the manipulator move time (and therefore mission time) can be quite costly and results in decreased work efficiency.
Various attempts to reduce the unwanted vibration or oscillation have been made. For example, U.S. Pat. No. 5,594,309 to McConnell et al. takes advantage of the system""s natural response to step inputs. A three step method is utilized to achieve maximum response with minimum time, while being open loop adaptive to changing system characteristics that are determined during its response.
Another approach was taken in Singer, N. C., Seering, W. P., xe2x80x9cPreshaping Command Inputs to Reduce System Vibration,xe2x80x9d Transactions, ASME, Vol. 112, March 1990, p. 76-62 (xe2x80x9cSinger methodxe2x80x9d). The Singer method consists of preshaping an open loop command sequence in order to achieve the desired response. This method requires prior knowledge of the system""s characteristics to be effective. However, when the Singer method is taken to its limit, it takes twice as long as the method in U.S. Pat. No. 5,594,309 to achieve the same response.
Different variations of the Singer method have also been attempted. Hyde, J. M. and Cutkosky, M. R, refer in xe2x80x9cControlling Contact Transition,xe2x80x9d IEEE Control Systems, February 1994, p. 25-30 (the xe2x80x9cHyde methodxe2x80x9d), an adaptation of the Singer method that uses preshaping to solve the xe2x80x9ctouchingxe2x80x9d problem that can occur at the end of the motion and set the robot arm into vibration. D""Amato, E., Di Gregorio, P., and Durante, F., refer in xe2x80x9cDynamic Modeling of Mobile Flexible Structures for Improvement of Motion Control,xe2x80x9d Proceedings, 12 International Modal Analysis Conference, Jan. 31-Feb. 3, 1994, Honolulu, P. 799-805 (the xe2x80x9cD""Amato methodxe2x80x9d) an application of the Singer method to a flexible cantilevered arm type of robot with limited success in its time response. It was found that in order to have rapid response, the robot structures need to be lighter and usually more flexible so that the robot""s mechanical vibration characteristics become more important. Many practical applications do not lend themselves to such limitations.
U.S. Pat. No. 5,049,797 to Phillips employs an arm tip deflection sensor. The feedback signal from this deflection sensor is used to reduce further deflection. Reducing arm deflection, however, reduces the force delivered to the payload and increases the overall system response time.
U.S. Pat. No. 4,925,312 to Onaga et al. refers to a control structure where the motor field torque is controlled by an inner loop and the position and velocity are governed by outer loops. This structure had previously been applied with some success to rigid robots. However, the Onaga structure is ineffective in controlling arm oscillations in flexible robots.
A preferred embodiment of the invention includes a control system (and corresponding method) which monitors and controls both joint torque and arm position during operation of a device such as a manipulator, whereby oscillation is rapidly removed when the system is disturbed by either movement, stopping of the system after completing a movement, by disturbance from external forces, or any other disturbance. Furthermore, the system can achieve rapid motion over a wide manipulator payload range. This is achieved in a preferred embodiment by providing control of torque and thereby allowing control of the arm deflection. In this illustrative arrangement, the joint torque takes on high levels (below the arm""s elastic limit), allowing rapid motion without causing arm oscillation. It has been found that the arm can be transformed from a cantilever-type structure to a pin-free-type structure. The invention allows for high joint torque levels with low frequency content, thereby enabling rapid arm movement without induced oscillation. The invention also provides for rapid arm stabilization in the event that the arm comes into contact with external disturbances.